The use of alkanolamines for removing acid gases such as CO.sub.2, H.sub.2 S and COS from natural and synthetic gas streams has been an industrial process for many years. The process is characterized by contacting the gas stream in an absorber or contact column with an alkanolamine, which is generally classified as a chemical solvent. The acid gases are removed from the gas stream by reaction with the solvent. In a continuous operation, the solvent is then sent to a desorber, or stripping column. The acid gases are separated from the solvent in a desorber by reducing the partial pressure of the acid gas over the solution and/or stripping the chemical solvent with steam. The steam is normally produced from the chemical solvent itself by heating the solvent in a reboiler. The lean solvent is then recycled back to the absorber and the process is repeated.
Although many improvements have been made in this process, the general concept of absorption followed by desorption in a cyclic manner has not changed. There have also been many improvements in the type of alkanolamines used in this process. In this regard, it is well known that most alkanolamines will slowly degrade under process conditions. As the alkanolamine degrades, it produces degradation products which are detrimental in that they reduce the absorbtivity of the chemical solvent, cause solvent foaming, and/or contribute to a corrosion of the process equipment. This is particularly true of primary and secondary alkanolamines used to remove carbon dioxide and/or carbonyl sulfide.
Initially, monoethanolamine (MEA), a primary alkanolamine, was used almost exclusively in the removal of acid gases. To prevent excess corrosion, the MEA concentration in the solvent is kept relatively low. This requires high solvent circulation rates, resulting in high energy requirements. In order to prevent corrosion and poor performance, the solvent must be periodically replaced or purified to remove these degradation products.
Purification usually involves the continuous thermal distillation of a small side-stream of the MEA. This solvent reclaimer, as it is called, maintains the MEA in an acceptable operating condition, but requires an additional amount of energy to operate. The bottoms from the reclaimer also represent a significant loss of MEA as well as a hazardous waste that is difficult to dispose of. This adds substantially to the operating expense.
As energy and capacity requirements increased, another solvent, i.e. diethanomlamine (DEA), began to be used. Since DEA is a secondary alkanolamine, it is more stable, less reactive, and potentially less corrosive than MEA, and can be used in higher concentrations. This increases the capacity of the solvent and decreases the overall energy requirement. However, DEA degradation is well known and is still a serious problem. Its degradation products have also been associated with the corrosion of process equipment. Thermal reclaiming of DEA is more difficult than with MEA due to its higher boiling point which is similar to that of many of its degradation compounds.
It is also known that tertiary amines, such as methyldiethanolamine (MDEA), do not undergo the same type of chemical and thermal degradation that primary and secondary amines undergo. As a result, tertiary amines are much more stable in gas treating processes, and MDEA is becoming increasingly acceptable as a replacement for MEA and DEA. Not only is MDEA more stable than MEA and DEA, it is potentially less corrosive and can be used in even higher concentrations. Higher amine concentrations also mean increased capacity and lower energy requirements. Unfortunately, the reactivity of CO.sub.2 with MDEA is much slower than with MEA and DEA. As a result, MDEA alone cannot be used in some applications which require almost complete CO.sub.2 removal. To get around this problem, DEA or MEA are sometimes added to MDEA to improve solvent reactivity. Such solvent mixtures have increased capacity and lower energy requirements than MEA or DEA by themselves, but are less stable than MDEA alone. Solvent degradation can make the benefits of using MDEA/MEA or MDEA/DEA mixtures uneconomical.